Abstract:

A study has been made of mass transport in electrolytic cells with vertical electrodes separated by a small distance. At the cathode, ferricyanide was reduced to ferrocyanide; at the anode, ferrocyanide was oxidized to ferricyanide. The mass transport rate was determined by measuring the steady-state limiting current ig. Under the conditions of our experiments ig was approximately inversely proportional to the distance a between the electrodes when a was smaller than about 0·4-0·6 mm. This corresponds to the theoretical relationship for mass transport without convection. At electrode distances larger than about 0·4-0·6 mm, natural convection becomes effective and the limiting current soon becomes virtually independent of a, as it is the case for the well investigated fully developed natural convection at a vertical plate where there is no interference by a counter-wall. The transition between the two transport regimes takes place mainly within a rather narrow range of a. The values of a at which the bending of the log ig/log a lines occurs depend on the electrode height and on the concentration of the reacting species. In addition, there is a broader transition region (a between 1 and 6 mm) where natural convection is unstable and fluctuations of the current with time are observed. © 1968.